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SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.1)
// ----

// Solution: change .button extendable to %button
%button {
  background: green;
  color: white;
  padding: 20px;
  font-size: 12px;
}
// Then just have a .button class @extend %button. This way html 
// elements with .button still get expected styles
.button {
  @extend %button;
}

// All instances of "@extend .button" need to be changed to "@extend %button"
.thingy1 {
  @extend %button;
}
.thingy2 {
  @extend %button;
  font-size: 48px;
}

// So now styling against .button no longer blows everything up
.something .button {
  font-weight: bold;
}
.another-thing .button {
  padding: 10px;
}

SassMeister-output.css

.button, .thingy1, .thingy2 {
  background: green;
  color: white;
  padding: 20px;
  font-size: 12px;
}

.thingy2 {
  font-size: 48px;
}

.something .button {
  font-weight: bold;
}

.another-thing .button {
  padding: 10px;
}